The present application relates to a hologram reproducing apparatus.
In recent years, along with attainment of high density recording, a hologram recording and reproducing apparatus, which is a recording and reproducing apparatus capable of recording and reproducing recording data at high transfer speed, attracts attention. The hologram recording and reproducing apparatus makes use of a thickness direction of a recording medium in addition to a width direction thereof and, in recording of data, forms interference fringes of reference light and signal light on the basis of page data corresponding to the recording data in a form of a hologram (a diffraction grating) in a hologram recording medium with two-dimensional information as a page unit and records the page data three-dimensionally. In reproduction of the recording data, the hologram recording and reproducing apparatus irradiates the reference light on the hologram formed in this way and obtains diffractive light generated by the irradiation to reproduce the recording data (see, JP-A-2004-226821 and “Nikkei Electronics”, Jan. 17, 2005, pp 106 to 114).
In recording of data, the hologram recording and reproducing apparatus displays two-dimensional information on a spatial light modulator (SLM) for each page and generates signal light and reference light to form a hologram. In reproduction of the recording data, the hologram recording and reproducing apparatus irradiates the reference light on the hologram to generate diffractive light and captures an electric signal corresponding to a reproduced image formed by this diffractive light with a two-dimensional imaging element represented by a CCD or CMOS image sensor. Therefore, there are a large number of sources of noise occurrence that are not present in optical disk recording and reproducing apparatuses in the past. A technique for reducing noise from such noise occurrence sources is necessary.
As a technique for noise reduction in a related technical field known in the past, there is a technique for noise reduction in the field of solid state imaging devices (see JP-A-2003-18475). This is a technique for outputting dark state data from a solid state imaging device using a light shielding jig in the manufacturing of a solid state imaging apparatus, outputting this dark state data to a CPU in the solid state imaging apparatus, detecting fixed pattern noise of the solid state imaging device from the dark state data with the CPU, compressing the detected fixed pattern noise and storing the fixed pattern noise in a nonvolatile memory, and, in actual use of the solid state imaging apparatus, moving information on the fixed pattern noise from the nonvolatile memory to a memory in an image processing circuit and storing the information therein, performing addition or subtraction of an imaging signal outputted from the solid state imaging device and the fixed pattern noise read out from the memory, and removing the fixed pattern noise from the solid state imaging signal.
However, a process of noise occurrence in the hologram recording and reproducing apparatus is different from a process of noise occurrence, in the solid state imaging apparatus. For example, stray light from an optical system has a significant influence on hologram reproduction as noise. When the hologram recording and reproducing apparatus performs multiplexing for forming plural holograms by sharing one recording area, crosstalk and the like among the holograms also have an extremely significant influence on hologram reproduction as noise in volume recording for recording plural holograms in one place. In a coaxial system for coaxially arranging signal light and reference light, light intensity of the reference light is larger than light intensity of diffractive light by about 2 to 3 digits. Thus, noise due to a contrast of the SLM (SLM contrast noise described later) occurs. Moreover, noise caused by the reference light leaking to an area in which the signal light is arranged (reference light leak noise described later) and the like are extremely significant problems.
Such noise due to the optical system has different scattering degrees of light beams depending on characteristics of hologram recording media. The influence of the noise is different for each of the hologram recording media and also different depending on characteristics of SLMs. Therefore, the influence is not uniform in each of hologram recording and reproducing apparatuses that use these optimal members. In this regard, a process of occurrence of noise is different from that of the fixed pattern noise in JP-A-2003-18475. Thus, it is difficult to obtain the effect of noise reduction with the technique disclosed in JP-A-2003-18475. When attention is paid to electric noise of a detection system that detects reproduced signal from diffractive light, in general, random noise, an output of which fluctuates as time passes, and fixed pattern noise appearing as steady output offset become noise sources at the time of signal reproduction and cause deterioration in an error rate. Consequently, a signal to noise ratio (SNR) of a reproduced image is deteriorated. In particular, when multiple recording is adopted, there is a conspicuous influence in that multiplicity is limited. However, a technique for removing the noise peculiar to holograms caused by the factors described above is unknown. From the viewpoint of ensuring compatibility when a hologram recording and reproducing apparatus and a hologram recording medium are appropriately combined and from the viewpoint of attaining high recording density using multiplexing and the like, a technique for removing, in reproducing a hologram, noise that occurs in hologram recording and reproduction is necessary.
Therefore, it is desirable to provide a technique for removing noise in hologram recording and reproduction.